Current transformers are low – power transformers whose primary windings are in the main circuit and whose secondary windings are practically short – circuited through the connected instruments, meters, relays or controllers. They isolate the instrument and protection circuit from the primary side and protect the devices against overload according to the transformers. Current transformers can have several secondary windings with cores of identical or different characteristics completely isolated, magnetically, from each other. Therefore for example, they can be fitted with two instrument cores of different accuracy classes or with instrument and protection cores having different overcurrent factors.